One prior art invention which relates to the present invention is described in U.S. Pat. No. 4,555,662, issued November, 1985, this patent disclosing a quantitative measurement method for inclusions, the method now being generally referred to as Liquid Metal Cleanliness Analysis (LiMCA for short). The LiMCA method and apparatus were originally developed for detecting nonmetallic inclusions during aluminium refining, but its application to iron and steel refining has also been investigated.
The LiMCA method is sometimes also referred to as the Electric Sensing Zone method (ESZ for short), the principle of the method being that when such an inclusion entrained in an electrically conductive fluid passes through an electrically-insulated orifice the electrical resistance of the fluid which is flowing through the orifice changes in proportion to the volume of the particle. The instantaneous change in the resistance is detected as a pulse in electrical potential between two electrodes on opposite sides of the orifice, and the number and size of the particles can be directly measured in the following manner.
First, if the particles are assumed to be spherical and of diameter d and the orifice is assumed to be cylindrical of diameter D, then the change R in the electrical resistance when a particle passes through the orifice is given by the following equation: EQU .DELTA.R=(4.rho.d.sup.3)/(.pi.D.sup.4) (1)
Where .rho. is the electrical resistivity of the fluid.
In actual practice, Equation (1) must be corrected by a correction factor F(d/D), which is given by the following equation: EQU F(d/D)=[1-0.8(d/D).sup.3 ].sup.-1 ( 2)
Thus, .DELTA.R is actually expressed by the following equation: EQU .DELTA.R=((4.rho.d.sup.3)/(.pi.D.sup.4).times.[1-0.8(d/D).sup.3 ].sup.-1 ( 3)
If the electric current through the orifice is I, then the pulse V in the electric potential when a particle of diameter d passes through the orifice is given by the following equation: EQU .DELTA.V=i(.DELTA.R) (4)
A previously-disclosed inclusion sensor probe which applies the above-described principles and intended for "continuous" use with molten metal (e.g. for periods as long as about 30-40 minutes) comprises an inner first electrode supported inside a quartz tube and connected to a water-cooled support. An orifice is provided in a portion of the quartz tube near to its lower end. The tube is mounted on the water-cooled support using a gasket to seal the joint between them. The necessary outer second electrode consists of a rod separate from the probe and extending close to the orifice.
When a measurement is to be performed the inside of the hollow electrode, which serves as a chamber to receive the molten metal, is evacuated and the molten metal is sucked inside through the orifice. At this time, the change in electric resistance between the inner and outer electrodes is measured and amplified by conventional means, and the sizes and number of inclusions are determined. When the tube is sufficiently full the negative pressure is replace by a positive pressure until the tube is empty and the cycle is repeated as many times as possible until the tube must be replaced.
The above-described sensor probe and others are used to perform "continuous" measurement by the LiMCA method in order to detect inclusions in molten aluminium and determine particle size distributions. Molten aluminium has a relatively low melting temperature of about 700.degree. C., so there are a number of different materials available from which the tube (heat resistant glass and quartz) and the electrodes (steel wire) can be made. However, the working temperatures of molten metal baths of metals like iron and titanium are much higher than for aluminium (above 1550.degree. C.), and at such temperatures there are considerable problems with lack of resistance of the probe and the electrodes to heat, so that it is difficult to employ these known sensors. There have been some applications of sensors of this type for measurement in the laboratory of inclusions of certain molten iron-silicon alloys having a temperature in the molten state of 1250.degree. C.
In order to apply the LiMCA method to molten steel and the like, it is necessary to solve the following problems.